A RFID system is an automatic identification system. A RFID reader recognizes an object through wireless communication with a tag that is attached to the object and that has an unique ID and information. Such a RFID reader must rapidly identify tags. As a reader and tags communicate through a single shared wireless channel, the signals thereof collide with each other. Such a collision interferes with the fast identification of tags or causes cases in which the reader cannot identify all objects.
As a result, in a RFID system including one reader and low-functional tags, anti-collision protocols which are capable of decreasing the frequency of collisions and rapidly identifying tags regardless of the occurrence of collisions is required.
Collisions are classified into reader collisions and tag collisions. The reader collisions are a confusion phenomenon occurring when neighboring readers simultaneously attempt to acquire information from a tag. The tag collisions are a phenomenon in which tags simultaneously try to respond to one reader, thereby preventing the reader from identifying the tags.
Reader collisions can be easily resolved because readers can detect collisions and communicate with one another. In contrast, since low-functional passive tags cannot determine the presence of neighboring tags or detect collisions, a tag anti-collision protocol significantly governs the identification ability of a RFID system.
Tag anti-collision protocols may be mainly categorized into an aloha-based protocol and a tree-based protocol.
The aloha-based tag anti-collision protocol employs a method in which tags select an arbitrary time and transmit their own IDs to a reader. This decreases the frequency of occurrence of tag collisions by decreasing the probability thereof, but cannot completely prevent the occurrence of collisions. Furthermore, a tag starvation phenomenon, in which a specific tag is not identified by a reader for a long time due to collisions, may occur.
The tag starvation phenomenon is a great disadvantage in an application system, such as logistics management, in which accuracy is important, therefore the aloha protocol is a representative probabilistic collision prevention protocol.
The tree-based tag anti-collision protocol, such as a binary tree protocol and a query tree protocol, employs a process of tracking the occurrence of collisions and detecting the existence of lags. The tree-based tag anti-collision protocol causes a long identification delay, but the tag starvation phenomenon does not occur.
The tree-based protocol divides a group of colliding tags into two sub groups, and then causes a reader to identify the IDs of tags without a collision therebetween. A binary tree protocol divides the group of colliding tags into two sub groups according to a randomly selected number using a counter with a random number generator. The tags which select 0 transmit their IDs. However, in this method, tags require a random number generator and a counter function.
Furthermore, in the query tree protocol, a reader transmits a query having a prefix several bits long, and a tag having an ID matched to a corresponding prefix responds. Colliding tags are classified according to the prefix of the reader. In this case, since the tags do not require additional memory other than memory for IDs, the query tree protocol is referred to as a memoryless protocol. In the query tree protocol, there is an advantage in that the function of a tag is simple, but there is a problem in that a long identification delay may be caused depending on the method of distribution of the IDs of tags which must be identified by a reader due to the use of prefixes.
Meanwhile, there are many modified protocols for the decrease of the long identification delay in the query tree protocol, but the frequency of the occurrence of collisions cannot be decreased merely by decreasing the time required for tag ID transmission.